1. Field of the Invention
This invention relates to a pedal crank drive for a bicycle, comprising a shaft extending through a bearing housing, a chain wheel having at least one gear rim, where the hub of the chain wheel is rotatably mounted with respect to the shaft, and comprising a power transmission spring supported on the one hand on the shaft and on the other hand on the hub of the chain wheel and disposed inside the bearing housing.
2. Description of the Prior Art
To obtain an advantageous torque distribution via a chain wheel rotation it is known (EP 0 546 004 B1) to provide between the chain wheel rotatably mounted with respect to the pedal crank shaft and the pedal crank adjacent the chain wheel a power transmission spring on which a load can be applied during a forward rotation of the pedal cranks, where part of the forces applied when treading down the pedal cranks can be stored in said power transmission spring. These stored forces can advantageously be delivered to the chain wheel in the vicinity of the dead center of the pedal crank drive, which involves a better torque distribution via a crank rotation. When the power transmission spring designed as helical spring is dimensioned sufficiently, so as to be able to absorb even major force components within the available, limited range of spring, there is the risk that via the power transmission spring the pedal cranks are rotated back in the dead center regions of the pedal crank drive against the supporting moment to be applied by the cyclist, when the power transmission spring, which is stretched by means of the pedal cranks leading when treading them down, is released again in the vicinity of the dead center. For this reason, there is used a damping spring prestressed in the loading sense of the power transmission spring, so that the power transmission spring can only be released against the force of this damping spring. Since the damping force of such a damping spring increases with increasing relief of the power transmission spring, there is obtained a more uniform torque distribution via a pedal crank rotation. However, the arrangement of the power transmission spring between the pedal crank adjacent the chain wheel and the chain wheel rotatably mounted with respect to the pedal cranks requires special constructive measures with regard to the large forces to be transmitted, which excludes a simple conversion of usual pedal crank drives without power transmission spring. In addition, the constructive modifications of a pedal crank drive, which are necessary when using a power transmission spring, involve a corresponding added weight.
To ensure that in the case of a bicycle with an auxiliary motor the auxiliary motor can automatically be activated in dependence on a predeterminable treading moment each applied via the pedal crank drive, it is also known (U.S. Pat. No. 5,024,286 A) to provide between the hub of a chain wheel rotatably mounted on the pedal crank shaft and the pedal crank shaft a helical spring wound about the pedal crank shaft, which helical spring provides for a rotation of the pedal crank shaft with respect to the chain wheel upon application of a corresponding treading moment, so that from the angle of rotation between pedal crank shaft and chain wheel a corresponding control of the auxiliary motor can be derived. However, this known construction can only be used for the control of an auxiliary motor, but not for an improvement of the torque distribution via a pedal crank rotation.